How Can I Seize Control of My Mother’s Finances From My Sister?

If your mother is still legally competent, she can revoke her existing durable power of attorney and execute a new one naming you or someone else as her new agent. If she does not have legal capacity, you will have to go to court to have a conservator or guardian appointed, who could then revoke the existing power of attorney.
For more on the power of attorney, click here.
For an article on revoking a power of attorney, click here.
For more on guardianship and conservatorship, click here.